A power supply of this type generally includes a line wire, a neutral or return wire, and a ground wire. In a single-phase system, the line wire is connected to the "live" supply terminal and carries a voltage of, usually, 110V; the return wire is theoretically at zero potential but is generally not connected to a local ground. In a three-phase system of the ".DELTA." type, in which these is no neutral lead, the line and return wires are constituted by different phase leads.
Ground-fault detectors are known which are designed to trip a circuit breaker whenever a dangerous leakage current develops as a result of faulty wiring. These conventional devices, however, protect only against one of several possible malfunctions which may threaten the safety of a human operator or may damage the equipment connected to the power supply.